scala> def foo[U](t: Any) = t.asInstanceOf[U]
foo: [U](t: Any)U
scala> val s: String = foo("hi")
scala> val n = foo("hi")
java.lang.ClassCastException: java.lang.String cannot be cast to scala.runtime.Nothing$
at .<init>(<console>:6)
at .<clinit>(<console>)
at RequestResult$.<init>(<console>:9)
at RequestResult$.<clinit>(<console>)
at RequestResult$scala_repl_result(<console>)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at scala.tools.nsc.Interpreter$Request$$anonfun$loadAndRun$1$$anonfun$apply$18.apply(Interpreter.scala:981)
at scala.tools.nsc.Interpreter$Request$$anonfun$loadAndRun$1$$anonfun$apply$18.apply(Interpreter.scala:981)
at scala.util.control.Exce...
有没有办法编写#foo,以便在未推断“U”或未显式设置为“真实”类型时返回 Any?
否。静态类型是U
。如果这被推断为Nothing
,编译器不允许返回类型的值Any
.
您可以改进运行时错误消息:
def foo[U: Manifest](t: Any): U = if (implicitly[Manifest[U]] == manifest[Nothing])
error("type not provided")
else t.asInstanceOf[U]
或者听从Arjan的建议。
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